1. Field of the Invention
This invention relates to a color cathode ray tube provided with a shadow mask.
2. Description of the Related Art
In general, a color cathode ray tube comprises an envelope and a substantially rectangular shadow mask. The envelope includes a panel that has a phosphor screen on its inner surface. The shadow mask is opposed to the phosphor screen in the envelope. A number of apertures are formed as electron beam passage apertures in a given array in a effective surface of the shadow mask that faces the phosphor screen. The shadow mask has the function of screening three electron beams that are emitted from an electron gun through the apertures so that the electron beams are incident upon three-color phosphor layers that constitute the phosphor screen.
Recently, flat tubes have become prevalent color cathode ray tubes. In order to reduce daylight reflection and image distortion and so improve visibility, a flat tube is designed so that the outer surface of its panel is substantially flat, having a curvature radius of 10,000 mm or more. Conventionally, the effective surface of the shadow mask that faces the phosphor screen is shaped corresponding to the shape of the inner surface of the panel. Therefore, the shadow mask of the flat tube is substantially flat, having a curvature smaller than that of a conventional color cathode ray tube.
However, use of a shadow mask with a small curvature involves the following problems.
Usually, the shadow mask is formed of a metal sheet with a thickness of about 0.2 mm. The shadow mask for a large screen that is formed of a thin sheet of this type is deformed by its own weight or external force, and cannot easily maintain its curved mask surface if the curvature of the effective surface is small. Thus, if the curvature of the effective surface is reduced, the retention of the curved mask surface (hereinafter referred to as curved mask surface strength) drops. The curved mask surface strength drops most significantly near the center of the effective surface or the picture center, in particular.
If the curved mask surface strength is low, the effective surface of the shadow mask is inevitably deformed by a very small external force during manufacture or transportation. In this case, the distance between the inner surface of the panel and the electron beam passage apertures of the shadow mask varies, so that the electron beams emitted from the electron gun cause a color drift, failing to land on predetermined phosphor layers.
Although the lowering of the curved mask surface strength never renders the shadow mask deformed, it inevitably causes the effective surface of the mask to be easily resonated by vibration such as sound when the mask is incorporated in a TV set. Thus, unwanted gradation is bound to appear on the picture.
Increasing the thickness of the shadow mask is the easiest method to prevent the curved mask surface strength from dropping. If the shadow mask thickness is increased, however, etching control for the manufacture of the shadow mask becomes difficult, and the variation of the diameter of the electron beam passage apertures worsens. In consequence, the yield of manufacture of the shadow mask and the color cathode ray tube and the picture quality level are lowered.
As means for solving these problems, therefore, shadow mask structures are described in Jpn. Pat. Appln. KOKOKU Publication No. 6-50610, Jpn. Pat. Appln. KOKAI Publication No. 2-123645 (Jpn. Pat. Appln. Publication No. 2743406), etc. They are constructed in a manner such that a plurality of shadow mask plates in the same shape, having electron beam passage apertures each, are superposed on one another and welded in a plurality of positions.
In the shadow mask constructed in this manner, a plurality of shadow mask plates are superposed closely on one another, whereby the plate thickness is simulatively increased to enhance the curved mask surface strength. If the degree of fixation of the shadow mask plates (hereinafter referred to as the lamination strength) is low, therefore, the degree of their adhesion lowers, so that the curved mask surface strength cannot be enhanced with ease.
If the lamination strength is low, moreover, the shadow mask plates are inevitably dislocated from one another when they are subjected to external force during manufacture or transportation. Thereupon, the electron beam passage apertures that are formed in the individual shadow mask plates are dislocated, so that the apertures, the electron beam passage apertures, are narrowed, and sometimes may be closed. In this case, the electron beams that pass through the apertures are reduced in number, so that light emission from the phosphor screen drops. In consequence, the luminance of images is partially lowered.